1. Field of the Invention
This invention relates in general to the field of interferometry and, in particular, to a novel light source for spectrally controlled interferometry.
2. Description of the Prior Art
Laser interferometry is a highly successful technique for measuring optical surfaces such as lenses and flats. Its popularity can be attributed mainly to the use of common-path interferometer designs—a type of device in which the reference and test beams follow the same path inside the interferometer resulting in optical-system errors canceling out, which enables less expensive and more accurate measurements. However, common-path interferometers can be implemented only with laser sources because the need for non-zero optical path difference (OPD) requires a high temporal-coherence source and corresponding non-localized interference.
Another class of interferometers is built around temporally incoherent light sources (white light interferometry—WLI) and uses localized interference fringes for recovering the test surface profile. These interferometers are immune to the coherent noise present with laser sources but require careful balancing of the OPD so interference can take place in the measurement space. Such arrangements can be complex and prevent the use of common-path designs, therefore forfeiting its advantages.
As detailed in U.S. application Ser. No. 12/816,091, a new concept described as spectrally controlled interferometry (SCI) successfully combines both approaches and provides the advantages of common-path interferometry with those of WLI. SCI enables the formation of localized interference in an unbalanced OPD interferometer and can use, for example, the Fizeau common-path interferometer design in WLI mode. One of the major advantages of SCI is that existing instrumentation can be adapted to it simply by replacing the laser with a light source capable of proper spectral modulation.
In spectrally controlled interferometry, the time delay (and therefore the OPD) between the object and reference beams is manipulated by changing the spectral properties of the source. The spectral distribution is tuned to produce a modulation peak at a value of OPD equal to twice the optical distance between the object and reference arms of the Fizeau interferometer, for instance, thereby enabling the use of its common-axis configuration to carry out white-light measurements free of coherence noise. Unwanted interferences from other reflections in the optical path are also removed by illuminating the object with appropriate spectral characteristics. OPD scanning can be implemented without mechanical means by altering the source spectrum over time so as to shift the peak location by a predetermined scanning step between acquisition frames. The spectrum can also be controlled on a pixel-by-pixel basis to create a virtual surface that matches the profile of a particular sample surface. Therefore, the availability of an easily implemented spectrally controlled light source is of primary importance to the successful implementation of SCI.
Spectrally controlled interferometry's ability to produce localized fringes in a setup with non-balanced OPD is related to the period of the source's spectral modulation. In general, the higher the frequency of spectrum modulation, the further the fringes will form away from the reference surface. This property can be exploited by filtering out undesired wavelengths from a light source such as a light emitting diode (LED).
Conventional filtering systems are built around a slit monochromator design, such as illustrated in FIG. 1. A light source 10 (the monochromator's input slit) is positioned in the front focal plane of a collimating lens 12. The collimated light passes through a dispersive element, in this case a prism 14, and is refocused by the focusing lens 16 in the plane 18 of the resulting spectrum. The prism 14 disperses the light and the irradiance detected in the spectrum plane is the result of convolution of the source's spectrum with the width of the light source 10. The irradiance is described by the equation:Islit(λ,y′)=Is(y′)*Id(λ),  (1)where λ is the wavelength, y′ is the coordinate along the spectrum plane, I is light irradiance, * is the convolution operator, Islit is the irradiance distribution in the spectrum plane, Is(y′) is the image of the slit through the spectrometer, and Id(λ) is the dispersion function of the prism.
Equation 1 shows that the ability to discriminate the spectrum is inversely proportional to the width of the light source 10 (the input-slit width). Shrinking the width of the light source, however, reduces the amount of light that enters the system and the image progressively becomes dimmer. This problem has been studied well in classical spectroscopy and is considered a fundamental limitation of this type of device.
Using the slit spectrometer arrangement of FIG. 1 as the basis for a simple spectrally modulated light source, a spatial light modulator (SLM) is placed in the spectrum plane 18. The SLM's active area is divided into individually controlled pixels with transmission or reflection properties that can be programmatically changed (see, for example, the SLM sold by Holoeye Photonics AG; http://www.holoeye.com/spatiallight_modulator_lc—2002.html). This feature allows the spectrum to be filtered by selectively blocking or attenuating parts of the SLM's active area. Simple arrangements are shown in FIG. 2 and FIG. 3, for example. In both arrangements the dispersive part of the system is essentially identical to the slit spectrometer shown in FIG. 1. In FIG. 2 the SLM is placed coincident with the spectrum plane 18 and a light pipe 20 is used to recombine the filtered spectrum and deliver it to the output plane 22, which can then be used as the source for SCI. In FIG. 3 the filtered light from the spectrum plane 18 is recombined by using a symmetrically placed spectrometer system (parts 26, 28 and 30). A lens 24 is used as a collector to increase light efficiency. The filtered light is recombined in the output slit 22 that can be used as the source in SCI. In essence, the exit 22 of such device forms an extended light source with spectral energy density controlled via an SLM or an equivalent device, such as a grating, placed on the spectrum plane 18. A reflective or transmissive SLM can be accommodated by small changes in the setup.
The basic problem with such arrangements is the trade-off between the spectral resolution and the amount of useful energy in the spectrally filtered beam. The efficiency of such devices can be increased by using a high-spatial coherence source with low temporal coherence, such as a superluminescent diode. Such devices have very high intensity (expressed in W/sr*m2), comparable to lasers, but a wide emission spectrum, comparable to LEDs. See, for example, the superluminescent diode sold by Qphotonics, LLC (http://www.qphotonics.com). However, such devices need a similar level of control as laser diodes (LD) for both current and temperature, which requires expensive electronics and heat sinks. Also, their emission spectra are typically located in the far-red spectrum and infrared, making them less useful for interferometry where the typical operating wavelength is 632.8 nm.
In view of the foregoing, a light source that overcomes these problems would be very desirable and would represent a significant advance in the art because it would facilitate the implementation of spectrally controlled interferometry with its shown advantages of both laser and white-light interferometry.